Peppers belong to the genus Capsicum, of the nightshade family, Solanaceae (e.g., Capsicum annuum). The term “pepper” may refer to the plant as well as its fruit. Peppers are commonly broken down into three groupings: bell peppers, sweet peppers, and hot peppers. Most popular pepper varieties fall into one of these categories, or as a cross between them. However, these groupings are not absolute, as both “hot pepper” and “sweet pepper” encompass members belonging to several different species. Additionally, members of each of the groups may be different cultivars of the same species. For example, the bell pepper, the jalapeno pepper, and the “Thai sweet” all belong to the species Capsicum annuum L. Hot peppers, including some inedible varieties, are grown for edible as well as ornamental and medicinal uses. While there are pungent (i.e., “hot”) varieties of C. annuum, many well known hot peppers are members of different species. For example, both the cayenne pepper and the tabasco pepper are varieties of Capsicum frutescens, while the hottest peppers, including the habanero and naga varieties, are members of Capsicum chinense. 
Pepper breeding efforts have focused in part on growing pepper plants resistant to diseases such as Bacterial Spot (BS), caused by the bacterium Xanthomonas campestris (“Xa”), including X. campestris pv. vesicatoria (“Xcv”). Bacterial Spot is one of the most important diseases of Capsicum sp. and causes necrotic spots on leaves, as well as premature leaf drop leading to sunscald of fruit, and spotting of stems and pods that results in unmarketable fruit. Leaf symptoms appear first on the undersides of leaves as small water-soaked areas. These spots may enlarge, turn dark brown, and are slightly raised. On the upper leaf surface the spots are depressed with a brown border around a beige center. Several lesions may coalesce, resulting in large necrotic areas, and large numbers of lesions can occur on leaf margins and tips where moisture accumulates. The disease is found worldwide and especially affects plants growing under warm moist conditions. Several classically defined resistance loci are known, including the resistance gene “Bs2”, originating from Capsicum chacoense accession PI 260435, which provides resistance to X. campestris races 0, 1, 2, 3, 7, and 8, i.e., those races of the pathogen that contain the avirulence gene avrBs2.
Disease management and prevention requires use of pathogen-free seed, crop rotation, and/or spraying with antibacterial agents such as streptomycin and copper-containing compounds. Because the disease can spread rapidly under optimum conditions and greatly affect the yield and marketability of a pepper crop, the development of new pepper varieties having resistance to Bacterial Spot, and for rapid and efficient methods for identifying Bacterial Spot-resistant pepper plants, would represent a significant advance.